The 3rd-generation (3G) Wideband Code-Division Multiple Access (W-CDMA) wireless network specified by the 3rd-Generation Partnership Project (3GPP) includes support for multiple-input multiple-output (MIMO) transmission techniques. (For details, see “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD) (Release 8),” 3GPP TS 25.214, available at http://www.3gpp.org/ftp/Specs/html-info/25214.htm.) In systems built according to these standards, a 2×2 MIMO scheme may be used to transmit the High-Speed Downlink Shared Channel (HS-DSCH) over two transmit antennas via two distinct spatially multiplexed data streams. The two streams use the same channelization codes, but are separated from each other by orthogonal preceding weights.
Because of imperfections in the radio propagation channel between the transmitting base station and a mobile terminal, the two streams will interfere with each other. This interference is referred to as code reuse interference. For optimal performance, a MIMO receiver needs to suppress or cancel this interference. In addition to suppressing code reuse interference, a MIMO receiver also needs an estimate of the code reuse interference power to compute accurate channel quality reports for feeding back to the base station. If the receiver computes channel estimates based on pilot channel symbols (e.g., the W-CDMA Common Pilot Channel, or CPICH), the ratio of the traffic channel power (e.g., the W-CDMA High-Speed Physical Downlink Shared Channel, or HS-PDSCH) to the pilot channel power, per channelization code must be known or estimated. This per-code traffic-channel-to-pilot power ratio αPC is used when suppressing or cancelling the code reuse term and may also be used to calculate an estimate of the received signal-to-interference-plus-noise ratio (SINR) for channel quality reporting.
One approach to suppressing code reuse interference in a Generalized Rake (G-Rake) receiver is described in U.S. Patent Application Publication No. 2008/0152053, titled “Method and Apparatus for Determining Combining Weights for MIMO Receivers” and published 26 Jun. 2008, the entire contents of which are incorporated by reference herein. With this approach, a receiver uses scaling parameters representing the normalized per-code energy allocated to each transmitted stream to calculate combining weights that suppress the cross-stream interference. These same scaling parameters may also be used to calculate the estimated code reuse interference power for the purposes of preparing channel quality reports.
Techniques for estimating the per-code traffic-channel-to-pilot power ratio αPC in a MIMO system are disclosed in U.S. patent application Ser. Nos. 12/036,425 and 12/036,368, both of which were filed Feb. 25, 2008 and both of which are titled “Code Power Estimation for MIMO Signals.” The entire contents of both of these applications are incorporated by reference herein. However, these or other previously known techniques may be unnecessarily complex, in some situations, or may overestimate αPC, or may yield excessively noisy estimates for αPC.